An automatic optosensing device for the simultaneous determination of resveratrol and piceid in wines

For the first time, a spectrofluorimetric method is reported for the simultaneous determination of resveratrol (RVT) and piceid (PCD), two stilbenes showing diverse interesting physiological and biochemical attributes, as well as a wide range of health benefits ranging from cardioprotection to chemoprevention. The method makes use of a multicommutated flow-through optosensor in which the resolution of RVT and PCD is accomplished by means the sequential arrival of their photoproducts, on-line generated by UV-irradiation, to the detection area. This is possible due to the different kinetic behaviour of these latter on a solid support (C₁₈ silica gel) filling a minicolumn placed before the detector. The measurement in solid-phase of the photochemically induced fluorescence of the photoproducts (λ_ex: 257 nm/λ_em: 382 nm) is used as analytical signal for monitoring both compounds. The method has been applied to the analysis of RVT and PCD in wines and requires a previous solid-phase extraction (SPE) using Bakerbond C₁₈ cartridges. This pretreatment and the use of a solid-support in both the minicolumn and the flow-cell of the detector allow the determination of RVT and PCD by external calibration. Detection limits (DLs) are 9.3 and 12.6 ng mL⁻¹ for RVT and PCD, respectively. Commercial red and white wine samples have been analysed and the results obtained have been satisfactorily validated by high-performance liquid chromatography (HPLC).     

Determination of azoxystrobin residues in grapes, musts and wines with a multicommuted flow-through optosensor implemented with photochemically induced fluorescence

In this paper, the conversion of azoxystrobin in a strongly fluorescent degradation product by UV irradiation with quantitative purposes and its fluorimetric determination are reported for the first time. A multicommuted flow injection-solid phase spectroscopy (FI-SPS) system combined with photochemically-induced fluorescence (PIF) is developed for the determination of azoxystrobin in grapes, must and wine. Grape samples were homogenized and extracted with methanol and further cleaned-up by solid-phase extraction on C₁₈ silica gel. Wine samples were solid-phase extracted on C₁₈ sorbent using dichloromethane as eluent. Recoveries of azoxystrobin from spiked grapes (0.5-2.0 mg Kg⁻¹), must (0.5-2.0 μg mL⁻¹) and wine (0.5-2.0 μg mL⁻¹) were 84.0-87.6%, 95.5-105.9% and 88.5-111.2%, respectively. The quantification limit for grapes was 0.021 mg Kg⁻¹, being within European Union regulations, and 18 μg L⁻¹ and 8 μg L⁻¹ for must and wine, respectively.     

Development of a photochemically induced fluorescence-based optosensor for the determination of imidacloprid in peppers and environmental waters

A novel flow-through solid phase spectroscopic assembly implemented with photochemically induced fluorescence (PIF) has been developed for the rapid, sensitive and selective determination of imidacloprid. The pesticide is derivatized on-line by irradiation with ultraviolet light providing an intensively fluorescent compound. The determination is carried out by measuring the fluorescence intensity of the photoproduct once retained on C₁₈ silica gel filling the flow-cell. The method proposed has been applied to the determination of imidacloprid in natural waters and peppers. The quantification limit (QL) (0.015 mg kg⁻¹) in peppers was lower than the maximum residue limit (MRL) established by Spanish Legislation (0.5 mg kg⁻¹). The method also seems to be suitable for environmental water analysis providing satisfactory recoveries (94.0-108.6%). The results obtained in the analysis of real samples are in good agreement with those provided by a reference liquid chromatography (HPLC) method.     

Multicommuted flow-through fluorescence optosensor for determination of furosemide and triamterene

Multicommutation implemented with flow-through optosensors is a very promising area of research. This recent approach benefits from the advantages of both methods and results in high sensitivity, selectivity, and speed, and little waste generation. This paper reports the simultaneous determination of furosemide and triamterene, two widely used diuretics, by measurement of their native fluorescence. The system has been proved to be useful for determination of both analytes in pharmaceutical preparations and for determination of triamterene in human urine and serum. A minicolumn filled with Sephadex SPC-25 microbeads was used to achieve separation of both analytes before detection in a flow-through cell filled with the same resin. The sensor is linear in the range 50–1200 and 0.4–8 ng mL⁻¹ with detection limits of 15 and 0.1 ng mL⁻¹ for furosemide and triamterene, respectively.     

A multicommuted fluorescence-based sensing system for simultaneous determination of Vitamins B2 and B6

A multicommuted flow-through optosensor based on the direct fluorescence measurements of Vitamins B₂ and B₆ using a non-polar sorbent (C₁₈ silica gel) as solid sensing zone (to accomplish the separation and subsequent preconcentration/detection of the target analytes) have been developed. The proposed flow system was controlled by Java-written home-made software and designed using three-way solenoid valves for independent automated manipulation of sample and carrier solutions. The native fluorescence signal was simultaneously monitored at two pairs of excitation/emission wavelengths (450/519 and 294/395 for B₂ and B₆, respectively). The separation of the analytes was performed in the detection flow cell, using the differences in the sorption/elution process on the solid support between the two vitamins, due to their different polarity. Using an optimised sampling time, the analytical signal showed linearity in the range 0.01-0.4 and 0.15-3 μg ml⁻¹ with detection limits of 0.003 and 0.045 μg ml⁻¹ for B₂ and B₆, respectively, obtaining R.S.D. (%) values better than 2% for both analytes. The proposed methodology was applied to different pharmaceutical preparations, obtaining remarkably good results with recoveries ranging from 96 to 107.5%.     

Flow-through optosensor combined with photochemically induced fluorescence for simultaneous determination of binary mixtures of sulfonamides in pharmaceuticals, milk and urine

A sensitive and selective flow-through optosensor implemented with photochemically induced fluorescence (PIF) is proposed for the simultaneous determination of mixtures sulfamethoxazole/sulfanilamide and sulfathiazole/sulfanilamide. The resolution was accomplished by placing in the flow system a minicolumn filled with an appropriate solid support. Whereas one of the sulfonamides is not retained in the minicolumn and is determined by measuring its native fluorescence on the solid surface of the sensing microbeads in the detection area, the other one is retained and, after its elution, it is photochemically converted into a strongly fluorescent photoproduct which is transitorily retained on the sensing support in the flow cell and monitored. Linear calibration graphs were obtained over a concentration range of 2–3 orders of magnitude. The detection limits for the determination of sulfamethoxazole, sulfanilamide and sulfathiazole are 8.1, 2.9 and 5.7 ng mL⁻¹, respectively. The method was applied to pharmaceuticals, milk and human urine. The recovery of sulfamethoxazole from pharmaceuticals was 102.5% indicating no interference from trimethoprim which is not photochemically active. The recoveries for urine and milk samples fortified with sulfonamides at levels between 0.1 and 0.7 μg mL⁻¹ agreed within 95.0–107.5% of spiked levels.     

A multicommuted flow system with solenoid micro-pumps for paraquat determination in natural waters

A flow system designed with solenoid micro-pumps is proposed for the determination of paraquat in natural waters. The procedure involves the reaction of paraquat with dehydroascorbic acid followed by spectrophotometric measurements. The proposed procedure minimizes the main drawbacks related to the standard chromatographic procedure and to flow analysis and manual methods with spectrophotometric detection based on the reaction with sodium dithionite, i.e. high solvent consumption and waste generation and low sampling rate for chromatography and high instability of the reagent in the spectrophotometric procedures. A home-made 10-cm optical-path flow cell was employed for improving sensitivity and detection limit. Linear response was observed for paraquat concentrations in the range 0.10–5.0 mg L⁻¹. The detection limit (99.7% confidence level), sampling rate and coefficient of variation (n = 10) were estimated as 22 μg L⁻¹, 63 measurements per hour and 1.0%, respectively. Results of determination of paraquat in natural water samples were in agreement with those achieved by the chromatographic reference procedure at the 95% confidence level.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

A novel flow-through fluorescence optosensor for the sensitive determination of tetracycline

A flow-through fluorescence optosensor with Sephadex G-50 microbeads as solid support is developed for the sensitive determination of tetracycline (TC). The fluorescent TC derivative encapsulated in CTAB micelle structures is retained onto the surface of the microbeads packed into a fluorescent flow cell in a flow system, followed by measurement of the native fluorescence of the TC derivative on the bead surface. The retained TC derivative is easily stripped off with DI water from the bead surface by breaking-down the micelle structure. This offers a convenient and effective way for the regeneration of the used solid support with DI water as a carrier. Under the optimal conditions, a linear calibration graph is obtained within a range of 3-500 μg L⁻¹, along with a detection limit of 1.0 μg L⁻¹. The present solid surface fluorescence optosensor provides a 22-fold improvement on the detection sensitivity for TC in comparison with that derived by fluorescence detection in aqueous medium. The feasibility of this flow-through fluorescence optosensor is evaluated by analyzing TC in a commercial drug tablet and surface water samples.     

Development of a rapid and automatic optosensor for the determination of cromolyn in biological samples

Disodium cromoglycate (SCG) is an anti-allergic drug, which is applied locally or inhaled. After administration, a very small portion of the drug is absorbed, being the most eliminated part unchanged in the urine and bile; therefore, its determination in urine is indicative of the dose absorbed. Here, the first spectroscopic method for the determination of SCG, making use of a sequential injection optosensor with terbium-sensitized luminescence detection, is described. The cationic resin Chelex-100 was used as solid support in the detection area. The measurements were made at 336/545 nm (λ_ex/λ_em) and the system was calibrated for two sample volumes, 150 and 800 μl, depending on the samples analyzed. A detection limit of 15 ng ml⁻¹ and a RSD lower than 2% (n = 10) were observed using the highest sample volume. The proposed method does not use any organic solvent or surfactant, so being environmental friendly. The analyte was satisfactorily determined in pharmaceuticals and human urine, the latter being spiked at the concentrations found after the administration of the drug.     

Photoinduced fluorimetric determination of folic acid and 5-methyltetrahydrofolic acid in serum using the kinetic evolution of the emission spectra accomplished with multivariate second-order calibration methods

Second-order multivariate calibration methods in combination with a continuous flow system, which allows for the continuous on-line irradiation of the analytes, have been employed for the determination of folic acid and its main metabolite 5-methyltetrahydrofolic acid in serum samples. An experimental central composite design, together with response surface methodology, has been used to find the optimum instrumental variables to perform the photochemical reaction. The time evolution of the emission spectra of the generated photoproducts, in the range 330-540 nm, after irradiation at 275 nm for 20 min, provided the three-way data set employed. On the basis of the differences on the kinetic rates of the photoreaction of both analytes, direct determination of the compounds in human plasma has been accomplished. The second-order methods assayed were parallel factor analysis (PARAFAC), self-weighted alternating trilinear decomposition (SWATLD), and unfolded partial least-squares (U-PLS), multidimensional partial least-squares (N-PLS), and bilinear least-squares (BLLS), all three in combination with the residual bilinearization procedure (RBL).     

A multicommuted flow-system for spectrophotometric determination of tannin exploiting the Cu(I)/BCA complex formation

A flow system exploiting the multicommutation approach is proposed for spectrophotometric determination of tannin in beverages. The procedure is based on the reduction of Cu(II) in the presence of 4,4′-dicarboxy-2,2′-biquinoline, yielding a complex with maximum absorption at 558 nm. Calibration graph was linear (r = 0.999) for tannic acid concentrations up to 5.00 μmol L^− 1. The detection limit and coefficient of variation were estimated as 10 nmol L^− 1 (99.7% confidence level) and 1% (1.78 μmol L^− 1 tannic acid, n = 10), respectively. The sampling rate was 50 determinations per hour. The proposed procedure is more sensitive and selective than the official Folin-Denis method, also minimizing drastically waste generation. Recoveries within 91.8 and 115% were estimated for total tannin determination in tea and wine samples.     

Coupled-column liquid chromatography method with photochemically induced derivatization for the direct determination of benzoylureas in vegetables

The coupled-column (LC-LC) system, consisting of a first column packed with internal surface reversed phase (ISRP) (50 x 4.6mm ID) and a Chrompack C18 (100 x 4.6 mm ID) as second column, allowed the simultaneous determination of five benzoylurea insecticides in dichloromethane (CH2Cl2) extracts of vegetable samples without any clean-up step. This system was combined with a photochemically induced fluorescence (PIF) post-column derivatization in order to provide strongly fluorescent photoproducts from the non-fluorescent benzoylureas. Limits of detection ranged from 0.21 to 0.98 microg L(-1) of pesticide (equivalent to 0.14-0.65 microg kg(-1) in vegetable samples) and limits of determination ranged from 4.0 to 10.0 microg L(-1) (equivalent to 2.7-6.7 microg kg(-1)). Linearity of the method was established between 2 and 1800 microg L(-1), depending upon the compound. Validation of the total method was performed by randomly analyzing recoveries of four vegetable samples (aubergine, cucumber, green bean, and tomato) spiked at two levels of concentration (10.0 and 33.3 microg kg(-1)). The combination of the LC-LC system with PIF detection provides a sensitive, selective, and rapid method for the determination of pesticides in vegetable samples at levels lower than the maximum residue levels (MRLs) established for these compounds by Spanish legislation.     

Coupled-column liquid chromatography combined with postcolumn photochemical derivatization and fluorescence detection for the determination of herbicides in groundwater

This study examines the application of coupled-column LC-photochemically induced fluorimetry-fluorescence detection (LC-LC-PIF-FD), demonstrating its potential for the quantitative and selective detection of six herbicides, including propanil and the phenylureas monuron, monolinuron, chlorotoluron, diuron and neburon in groundwater samples. An AQUASIL C18 50 x 4.6 mm(2) id column coupled to an AQUASIL C18 150 x 4.6 mm(2) id column for analyte clean-up and determination were used, respectively. A simple SPE with Cl8 cartridges was carried out, yielding average recoveries between 80 and 112% (n = 6) with RSDs between 0.5 and 9%. The LODs ranged from 0.0083 to 0.0833 microg/L in the groundwater samples.     

Application of coupled-column liquid chromatography combined with post-column photochemically induced fluorimetry derivatization and fluorescence detection to the determination of pyrethroid insecticides in vegetable samples

This study reports the first application of coupled-column liquid chromatography–photochemically induced fluorimetry–fluorescence detection (LC-LC-PIF-FD), demonstrating its potential for the quantitative and selective detection of seven pyrethroids in vegetable samples such as cucumber, green bean, tomato and aubergine. An internal surface reversed-phase (ISRP) column coupled to a C₁₈ column for analyte clean-up and determination were used, respectively. In comparison with a C₁₈ column, the ISRP substantially improved the separation between analytes and interferences from the vegetable matrix. The limits of detection ranged from 0.01 to 0.22 g kg^–1 in the vegetable samples (equivalent to 0.01 and 0.13 g L^–1 in the extract injected), and limits of determination ranged from 0.56 to 8.33 g kg^–1 in the vegetable samples (equivalent to 0.34 and 5.00 g L^–1 in the extract injected). Samples were extracted into dichloromethane to yield mean recoveries at two levels of concentration between 72.8 and 110.0% in all cases. Relative standard deviations were lower than 11%.     

A novel flow-through fluorescence optosensor for the determination of thiabendazole

This paper presents the development of a new flow-injection system combined with solid-surface fluorescence detection for the determination of the widely used fungicide thiabendazole. Nylon powder was probed as a novel solid support for building the optosensor. The method is based on the on-line immobilization of thiabendazole onto nylon in a continuous flow system, followed by the measurement of its native fluorescence. Aqueous samples are directly injected in a water carrier, resulting in a very simple and economical method. The analytical figures of merit obtained using 1500 μL of sample and 75% methanol (v/v) as eluting solution were: linear calibration range from 8 to 120 ng mL⁻¹ (the lowest value corresponds to the quantitation limit), relative standard deviation, 0.9% (n = 5) at a level of 64 ng mL⁻¹, limit of detection calculated according to 1995 IUPAC recommendations is to 2.8 ng mL⁻¹, and sampling rate of 14 samples h⁻¹. The potential interference from other agrochemicals, metal ions and common anions, and the viability of determining thiabendazole in real water samples were also evaluated.     

A green flow-based procedure for fluorimetric determination of acid-dissociable cyanide in natural waters exploiting multicommutation

A flow system designed with solenoid valves is proposed for determination of weak acid dissociable cyanide, based on the reaction with o-phthalaldehyde (OPA) and glycine yielding a highly fluorescent isoindole derivative. The proposed procedure minimizes the main drawbacks related to the reference batch procedure, based on reaction with barbituric acid and pyridine followed by spectrophotometric detection, i.e., use of toxic reagents, high reagent consumption and waste generation, low sampling rate, and poor sensitivity. Retention of the sample zone was exploited to increase the conversion rate of the analyte with minimized sample dispersion. Linear response (r = 0.999) was observed for cyanide concentrations in the range 1–200 μg L⁻¹, with a detection limit (99.7% confidence level) of 0.5 μg L⁻¹ (19 nmol L⁻¹). The sampling rate and coefficient of variation (n = 10) were estimated as 22 measurements per hour and 1.4%, respectively. The results of determination of weak acid dissociable cyanide in natural water samples were in agreement with those achieved by the batch reference procedure at the 95% confidence level. Additionally to the improvement in the analytical features in comparison with those of the flow system with continuous reagent addition (sensitivity and sampling rate 90 and 83% higher, respectively), the consumption of OPA was 230-fold lower.     

The potential of combining solid-phase optosensing and multicommutation principles for routine analyses of pharmaceuticals

In this work, we have explored the analytical potential of combining solid-phase optosensing and multicommutation principles, applied to the field of routine analyses of pharmaceuticals. This marriage benefits from the advantaging features of both concepts: the ability of multicommutation to provide increased repeatability, easier sample handling, reduced sample and reagent consumption as well as minor waste generation, combined with the enhancement of both sensitivity and selectivity obtained when a solid support is used to carry out the spectroscopic measurements directly on it. This approach has been evaluated by developing a method for the simultaneous analysis of two active principles (piroxicam and pyridoxine) in pharmaceutical formulations, using a non-polar sorbent as a solid support to attain the separation and subsequent preconcentration/detection of the targeted analytes. A multicommutated flow-through multisensor based on the direct intrinsic solid-phase UV absorbance measurements of the analytes on a packed C₁₈ silica gel bed was then thoroughly developed. The usefulness of this approach was assessed when it was applied to the determination of piroxicam and pyridoxine in different pharmaceutical formulations obtaining remarkable results.